The deployment of cryptography on sensor networks is a challenging task, given the limited computational power and resource-constrained nature of the sensoring devices. This paper presents the implementation of binary elliptic curves on the MICAz Mote. Optimization techniques for arithmetic algorithms on binary fields, including squaring, multiplication and modular reduction are presented. Our implementation of field multiplication and modular reduction algorithms focus on the minimization of memory accesses and appear as the most efficient algorithms published for this platform. This leads to an improvement of 39% over the best implementation for computing a point multiplication on a Koblitz curve. The results also show that binary elliptic curves can perform on this platform as well or better than elliptic curves defined over prime fields.